Use xf86ScreenToScrn and xf86ScrnToScreen

Part of the new compat-api.h are ScreenPtr <-> ScrnInfoPtr APIs.
Let's use them.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>

BUG=chromium:265459
TEST=builds clean; sanity check ui

Change-Id: I347f5973eae02066b9a6fc27e07599e519b95abf
Reviewed-on: https://gerrit.chromium.org/gerrit/65843
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Tested-by: Daniel Kurtz <djkurtz@chromium.org>
Commit-Queue: Daniel Kurtz <djkurtz@chromium.org>
7 files changed